A belated followup:
The Multipack is a good design. I only used it as a front pack for a larger pack, and it worked well. The supplied straps and cordlocks are a pretty flexible system, though if your shoulder straps are totally bereft of daisy chains or the like you might need to do some sewing. Tension can be adjusted easily via the side straps: tight for a more secure fit, loose for better air circulation. If anything, I found the size a bit much. My ideal load flopped around a bit, and the pack is big enough that it got in the way during more technical scrambling. Not a big deal, and it is big enough to hold a larger camera kit. The zipper requires two hands or quite a bit of fiddling to open and shut, which I found off putting.
A hole did appear near one of the upper corners after a month of use. The seam allowance was a bit small and eventually the threads pulled out. This was quickly and cheerfully replaced by Zpacks. I sold the replacement as I decided it didn't fit my needs.
I've been using a wearable (and much heavier, the HPG Runner's Kit bag) chest pack this winter, which has a wrap around zipper and several pockets. I like the organization, though making the Multipack like this would add weight and expense. I do think a different zipper orientation would make the Multipack easier to open and provide more flexibility for accessing contents from either the top or side. I'd also like to see Zpacks offer a wearable harness as an option, though it is pretty easy to make one yourself.
Overall, the Multipack is a solid iteration of a very good concept.